bufferSize

The `bufferSize` influences the size and frequency of reads.
For example, suppose we throttle a very fast origin source at 10K bytes per second,
if bufferSize=10K, read() will stall for 1 second then yield a 10K buffer;
if bufferSize=1K, read() will stall for 0.1 second then yield a 1K buffer.

Skipping

Skipped bytes are not counted as served bytes. The `skip(n)` method of this class
is guaranteed to return `n`, so that the skipped bytes will not interfere
with the throughput of read().